LCD TVs and monitors use backlights consisting of arrays of cold cathode fluorescent lamps (CCFLs) to create a visible image on the LCD. For large displays a direct backlight type is used, where the lamps are directly placed behind the LCD, as shown in FIG. 1A. FIG. 1A shows the CCFLs 10, a diffuser plate 12, and an LCD panel 14. Disadvantages of using CCFLs are that they require mercury, have a low color gamut, and have a limited brightness.
Alternative solutions have been proposed that use LEDs, which use either a waveguide and edge illumination, or a direct backlight with side emitting LEDs (i.e., U.S. application Ser. No. 10/442,346, assigned to Lumileds Lighting U.S. LLC). In both approaches, a long mixing length is created to deal with the flux and color variations that are inherent to LEDs. In the direct backlight approach, each LED illuminates a large area of the LCD, or, in other words, each pixel of the LCD is illuminated by a large number of LEDs such that variations in output of each LED do not show up in the LCD image. In the edge lit as well as the side-emitting direct backlight approach, the flux and color mixing properties come at an efficiency penalty.
In U.S. Pat. No. 6,582,103 B1 (to John Popovich et al, assigned to Teledyne Lighting and Display Products), low profile LED illumination fixtures are proposed, consisting of a cavity, including reflective walls, an output aperture, and at least one point source, such as an LED. In this patent, a diffuser covers the output aperture, and each LED includes a side-emitting lens. The solution presented in the present application does not require such side-emitting lens. Other distinguishing feature exist.
A complete other illumination approach was introduced by Whitehead et al. of the University of British Columbia in Canada (WO 02/069030 A2; SID 03 Digest, Helge Seetzen, Lorne A. Whitehead, A High Dynamic Range Display Using Low and High Resolution Modulators, p. 1450–1454), who proposed and demonstrated, as shown in FIG. 1B, an array of LEDs 16 directly behind the LCD 18. Only a few pixels 20 are illuminated by a single LED. The benefit of this approach is that the intensity of the LEDs can be modulated to represent the low spatial frequencies in the image, while the LCD modulates the high frequencies. The big advantage of this is that the dynamic range and contrast of the display are greatly enhanced (16 bit versus 8 bit displays). This is of great advantage in professional (e.g., medical) applications, but would create much better picture quality for an LCD display as well. One of the big challenges in this approach is the variation in color and flux of the LEDs. This is especially true if red, green, and blue LEDs are used to create white, but for white LEDs as well. Without a sufficient density of the LEDs, it will be very difficult to get adequate brightness uniformity with the configuration as suggested by Whitehead. Another disadvantage of this approach is the cost of the system. In the SID03 paper, it is suggested to place the LEDs at a pitch of 5 mm. For a 37″ diagonal LCD-TV, 16,000 LEDs would be required. Besides the cost, one has to cope with driver and connection reliability issues as well.
Another illumination approach where a high efficiency LCD display is obtained is disclosed by Mueller-Mach et al. in U.S. application publication US2002/0145685A1, assigned to Lumileds Lighting U.S., LLC. In this illumination scheme, a blue backlight is used in combination with a phosphor dot pattern consisting of red and green phosphor dots, which are aligned with LCD pixels representing the red, and green image pixels, respectively, while the blue pixels are left blank or applied with a non-phosphor scattering material. A related approach was suggested by Gallen et al. (WO 02/075440) where an UV or near UV emitting LED array was used, and red, green, and blue phosphors where applied (screen printed) onto the LCD. A collimating means is used to limit the cross-talk between the LCD pixels and the phosphor dots. Both applications have the advantage that the color uniformity is determined by the phosphor and the phosphor printing process, and that the system efficiency can be very high, as the absorbing color filters are no longer needed. However, efforts in this area have not yet resulted in introduction of this technology to the market.